This invention relates to thermoplastic, polyolefin materials (xe2x80x9cpolyolefinsxe2x80x9d) having superior mar, scratch, wear, and abrasion resistance and methods of making such polyolefins by exposure to radiation to crosslink unsaturated polyolefins therein.
Polyolefins are useful in a wide variety of applications due to their strength, environmental resistance and moldability. Many polyolefins are too easily scratched, marred, worm, abraded or otherwise damaged on their surface. These surface characteristics may be measured in a variety of ways. For example, scratch resistance may be measured by vertically penetrating a scratch needle, with a spherical tip into a polyolefin or other material surface under a constant load. The needle is then moved horizontally at a constant rate, and the width and/or depth of any formed scratch is measured (See, e.g., T. Nomura, et al., J. Applied Polymer Sci. 55:1307-1315 (1995)). Generally, it has been desired to enhance the service life of a polyolefin by improving these surface characteristics to reduce scratching, marring, wearing, abrasion, and the like.
One conventional method to enhance surface characteristics has been to use inorganic particulate material, such as various silicas. Uniform dispersion of these particulates is difficult to achieve, however, and this results in non-uniform surface properties in such products. The use of these particulates also tends to damage other desirable physical properties of the polyolefin, resulting in loss of impact strength, toughness, processability, and the like.
A more effective conventional method of providing surface enhancing characteristics to certain polyolefins is disclosed in U.S. Pat. No. 4,921,669. This patent discloses passing a web of extruded thermoplastic synthetic resin through a polishing roll stack, wherein at least one of the rolls has thereon a film of a material forming such a scratch resistant coating that is transferred from the roll to the web surface. Such conventional methods of enhancing the surface characteristics of a polyolefin require expensive, bulky equipment that also increase the processing time of polyolefin products.
Another way to enhance surface characteristics of polyolefins is described in U.S. Pat. No. 4,000,216, which discloses an extrudable, moldable, or heat formable blend of a thermoplastic polymer and a surface altering agent of at least one monoethylenically unsaturated monomer for said thermoplastic polymer, wherein the surface altering agent has cross-linked polymer particles having an average size of 1 to 30 m. The surface altering agent is preferably prepared by an endopolymerization, which is used with a compatible polyolefin to be altered.
Another conventional way to enhance surface characteristics of various articles is to apply acrylic polymers or coatings to an article and subsequently cure the polymer or coating with a radiation source, such as ultraviolet radiation (xe2x80x9cUVxe2x80x9d). The following patents describe a variety of examples of such conventional xe2x80x9capply coating and curexe2x80x9d methods and compositions.
U.S. Pat. Nos. 4,153,526 and 4,039,720 disclose safety glass made by laminating a saturated polyvinyl acetal film and a photoinitiator to a ply of glass, and irradiating the film with UV to initiate crosslinking and to provide improved solvent-abrasion resistance. U.S. Pat. No. 4,227,979 discloses radiation-curable coating compositions including one or more amide acrylate compounds that form mar-resistant protective and decorative film coatings.
U.S. Pat. No. 4,255,303 discloses a composition for coating electrical applications including ethylene polymer and at least 10 parts by weight of talc filler per 100 parts by weight of polymer, where the talc is coated with at least one metallic salt of a fatty acid having 8 to 20 carbon atoms. The ethylene polymers may be curable by irradiation with high-energy electron beams or a chemical curing agent, such as organic peroxide. U.S. Pat. No. 4,371,566 discloses actinic radiation curable coating compositions for application to many substrates having a pentaerythritol-based polyacrylate or polymethacrylate, such as pentaerythritol tetraacrylate, a vinyl chloride-vinyl acetate containing polymer, and a photoinitiator, preferably applied by spraying a solution onto the substrate.
U.S. Pat. No. 4,478,876 discloses a process of coating a solid substrate with an abrasion resistant silicone hard coating curable upon UV exposure under a non-inert atmosphere, such as air. The composition includes a UV crosslinkable polyfunctional acrylate monomer, SiO2 in the form of colloidal silica, and acryloxy functional silanes and a selected blend of ketone-type and hindered amine-type photoinitiators.
U.S. Pat. No. 4,814,207 discloses a method for applying a scratch and weather resistant film coating to a shaped article by applying to the article a thin film of a mixture of free radically polymerizable monomer having at least two polymerizable olefinic bonds, a peroxide initiator having a half life of less than two minutes at 100xc2x0 C., and an ultraviolet stabilizer, and curing the film by heating it to a temperature greater than 70xc2x0 C. U.S. Pat. No. 4,902,578 discloses a radiation-curable coating for thermoplastic substrates having a polyfunctional acrylic monomer, a mono-, di-, or trifunctional acrylic monomer, a thermoplastic or elastomeric polymer, and a photoinitiator.
U.S. Pat. No. 5,006,436 discloses a UV curable, aqueous alkaline developable solder mask composition having a thermal free radical initiator capable of generating free radicals with heat, and a polyunsaturated compound capable of being thermally crosslinked by the free radicals to provide a substantially fully cured coating. U.S. Pat. No. 5,316,791 discloses a process for improving the impact resistance of a coated plastic substrate by applying an aqueous polyurethane dispersion as a primer layer, partially curing the layer by air drying, applying a coating composition over the primer layer, and curing the coating composition to form an abrasion-resistant hard coat. U.S. Pat. No. 5,382,604 discloses a crosslinked adhesive composition having an epoxidized diolefin block polymer crosslinked by UV radiation through at least some of the epoxy functionality.
U.S. Pat. No. 5,558,911 discloses a method of coating articles with powder coatings having polymers applied to the substrate to be coated, melting the polymers, and crosslinking by UV radiation.
U.S. Pat. No. 5,591,551 discloses a lithographic coating and method of coating at least a portion of a surface allow of an article with a radiation-crosslinkable polymer and exposing it to a pattern of radiation to produce an image. The polymer is disclosed to be a copolymer of an isoolefin of 4-7 carbon atoms and para-alkylstyrene.
U.S. Pat. No. 5,618,586 discloses self-crosslinkable film-forming compositions as coatings and a process for preparing multi-layered coated articles with a colored base coat and a clear top coat, wherein the composition includes a non-gelled addition polymer that is the free radical initiated reaction product of an N-alkoxymethyl(meth)acrylamide and at least one other ethylenically unsaturated monomer.
These conventional curable products generally require several steps, particularly coating or effectively laminating a prepared product with the V-curable coating. However, it would be advantageous to provide independent polyolefin compositions that are capable of being cured without the need for additional processing steps, such as coating or laminating an article with a curable composition. It is thus desired to produce such a product, preferably one having enhanced surface durability. The present invention provides new polyolefin compositions that satisfy this need.
The present invention relates to a thermoplastic polyolefin composition having enhanced surface durability, which includes a base component of a thermoplastic polyolefin, at least one radiation-crosslinkable component present in an amount sufficient to enhance the surface durability of the thermoplastic polyolefin base component when radiation-cured, and at least one photoinitiator present in an amount sufficient to initiate crosslinking of the radiation-crosslinkable component upon exposure of the thermoplastic polyolefin composition to radiation.
In one embodiment, the polyolefin composition also includes a filler in an amount of between 0.1 to 30 weight percent. In a preferred embodiment, the filler includes calcium carbonate, clay, talc, mica, glass, zinc oxide, wollastonite, silica, titanium dioxide, or mixtures thereof.
In another embodiment, the thermoplastic polyolefin base component is thermally treated prior to or subsequent to radiation exposure. The thermal treatment may vary in intensity from 25xc2x0 C. to 150xc2x0 C. and may vary in duration from a few minutes up to a few days (e.g., from 2 minutes to 48 hours) depending on the temperature and desired extent of cure. Alternately, the base component can be subjected to a series of alternating thermal and radiation treatments which may be of varying duration lengths depending on the desired physical properties.
The compositions may contain photo-oxidative stabilizers that prevent environmental degradation of the thermoplastic polyolefin base component without interfering substantially with the photoinitiated surface crosslinking.
The thermoplastic polyolefin base component may include a blend of a crystalline or semicrystalline poly-xcex1-olefin and an amorphous poly-xcex1-olefin. In a preferred embodiment, the thermoplastic polyolefin base component includes a blend of semi-crystalline polypropylene and amorphous ethylene/xcex1-olefin copolymer. In another embodiment, the amorphous poly-xcex1-olefin includes amorphous copolymer of ethylene and at least one diene. Generally, the thermoplastic polyolefin base component is present in an amount of about 99 to 55 weight percent of the composition. In a preferred embodiment, the thermoplastic polyolefin base component is present from about 95 to 65 weight percent of the composition.
The radiation-crosslinkable component may include an unsaturated polyolefin. In a preferred embodiment, the unsaturated polyolefin includes an ethylene, propylene and diene terpolymer; polybutadiene; polyisoprene; a styrenic polymer; an ethylene and propylene copolymer; methacrylate grafted polybutadiene or mixtures thereof. Typically, the radiation-crosslinkable component is present in an amount of about 1 to 30 weight percent of the thermoplastic polyolefin composition.
The photoinitiator may be any one of benzoin and benzoin ether derivatives; benzil ketal derivatives; xcex1,xcex1-dialkyoxyacetophenone derivatives; xcex1-aminoalkylphenone derivatives; xcex1-hydroxyalkylphenone derivatives; acylphosphine oxides; acylphosphine sulfides; phenylglyoxylate derivatives; O-acyl-2-oximino ketone derivatives; benzophenone and its derivatives; Michler""s ketone and its derivatives; thioxanthone and its derivatives; mixtures thereof; and all the polymer-bound compounds of the type mentioned above. In a preferred embodiment, the photoinitiator is benzil ketal derivatives; and acylphosphine oxides. The photoinitiator is typically present in an amount of about 0.01 to 5 weight percent of the composition. Mixtures of photoinitiators may also be used, along with accelerators that facilitate rapid crosslinking.
The invention also relates to a method of providing a cured thermoplastic polyolefin composition having enhanced surface durability by preparing a mixture of a thermoplastic polyolefin base component, at least one radiation-crosslinkable component in an amount sufficient to enhance the surface durability of the thermoplastic polyolefin component when radiation cured, and at least one photoinitiator in an amount sufficient to facilitate crosslinking of the radiation-crosslinkable component when the thermoplastic polyolefin composition is exposed to radiation, and then exposing a surface of the thermoplastic polyolefin composition to radiation in an amount sufficient to crosslink the radiation-crosslinkable component at least at the exposed surface, thereby providing a thermoplastic polyolefin composition having enhanced surface durability.
In one embodiment, the radiation is selected to be ultraviolet, electron beam, gamma, visible, microwave, infrared, or thermal radiation, or mixtures thereof. In another embodiment the radiation is imparted at a power of 0.1 J/cm2 and above and at a wavelength between about 200 to 500 nm.
In a preferred embodiment, the thermoplastic polyolefin base component is selected to be a blend of a crystalline or semicrystalline poly-xcex1-olefin and an amorphous poly-xcex1-olefin, the radiation-crosslinkable component is selected to be an unsaturated polyolefin that is present in about 0.01 to 20 weight percent of the composition, the photoinitiator is selected to be benzil ketals or phosphine oxides present in,;about 0.1 to 5 weight percent of the composition, and a filler is added to the mixture. The method may further include forming the thermoplastic polyolefin composition into a desired shape prior to exposing the surface of the composition to the radiation.
The invention also relates to a thermoplastic polyolefin composition having enhanced surface durability formed by the above-described method. In one embodiment, the composition has an outer surface that contains a polymerized component so that the surface scratch resistance after irradiation has a rating of 1.5 or lower on the Solvay scale (see definition below) after cure.